Is that what you wanted?
I've created two calc measure One is for previous year month sales and second is current year month sales i.e. your month year selected from parameter. Here you can see I've selected Oct-12.
Oct-12 sales: 31405
Oct-11 sales: 31453
which is less then previous year so its down arrow as per your logic.
Go through the workbook. Let me know If you've any query.
Null issue in tableau.twbx 1.2 MB